1. Field of the Invention
This invention relates to a process for preparing a compound using a vanadium oxide derivative for the cathode of a thermal cell.
2. Related Art
Vanadium oxide, and more particularly, vanadium pentoxide, cannot be used directly as the active material of the cathode of a thermal cell because, on the one part, it is a poor electrical conductor and, on the other part, it is reactive in fused chloride media.
For those skilled in the art, improving the conductivity of cathodes by mixing this material with an electrical conductor such as graphite or carbon black may appear to be an obvious solution. However, when V.sub.2 O.sub.5 is mixed with carbon black and one attempts to use this compound in the cathode of a thermal cell, the resulting mixtures are unstable and display low conductivity. In a battery, the result always translates into a series of potential discharge stages. In addition, mixtures containing graphite or carbon black in quantities sufficient to improve conductivity are always difficult to pelletize.
Other methods have been proposed for preparing vanadium oxide derivatives which are suitable for use as the cathode materials of thermal cells.
More specifically, U.S. Pat. No. 4,315,905 proposes the creation of an electrically conductive product to be used as a cathode material or as a cathode coating material by heating vanadium oxide in a reducing or neutral gaseous atmosphere (hydrogenated nitrogen or helium). The resulting mixture contains both V.sub.6 O.sub.13 and an unidentified derivative of V.sub.2 O.sub.5, but no example for the use of this compound is provided.
European patent no. BE-A-145,261, dated Nov. 6, 1984 describes the use of a cathode material selected from the group comprising one or more of the following compounds:
V.sub.6 O.sub.13--x (O&lt;.times.&lt;0.5), VO.sub.2.+-.z (O&lt;z&lt;0.05), Li.sub.y V.sub.2 O.sub.5 (0.88 &lt;y&lt;1), V.sub.3 O.sub.7 and LiV.sub.3 O.sub.8.
However, the compounds described above are not suitable for use as a cathode material in a thermal cell. More particularly, they do not display a stable discharge voltage, as shown by the examples of discharge provided in the patent. Regardless of the compound employed, the discharge curves display a steep slope and a multiplicity of stages. In addition, the preparation process requires that several operations be performed in succession in order to produce the final cathode mixture containing the electrical conductor and a suitable quantity of electrolyte; the preparation time, which ranges from 8 hours to several days, is prohibitive.
The object of this invention is to implement a simple process for preparing a compound using a vanadium oxide derivative, wherein this compound is suitable for use as a cathode material in a thermal cell and displays a stable discharge voltage.
This object is achieved by a process for preparing a compound using a vanadium oxide derivative which is suitable for use as a cathode material in a thermal cell, and wherein:
a powdered mixture is prepared by combining vanadium pentoxide V.sub.2 O.sub.5, a carbon, in proportion to the V.sub.2 O.sub.5, ranging from 3 percent to 20 percent by weight inclusive, and a mixture of alkaline halides, in proportion to the V.sub.2 O.sub.5, ranging from 15 percent to 50 percent by weight inclusive;
heat processing is conducted at a temperature which is higher than the melting point of said mixture of halides, wherein said temperature ranges from 365.degree. C. to 650.degree. C. inclusive, for a time period ranging from 15 minutes to 2 hours inclusive.
Preferably,
said carbon is a gas black, such as carbon black, acetylene black or benzene black;
said mixture of alkaline halides is a mixture of lithium and potassium halides, selected from the group consisting of the eutectic LiCl-KCl, mixtures of LiCl-KCl containing from 50 mole percent to 80 mole percent of LiCl, and ternary mixtures of LiCl-LiBr-LiF, LiCl-LiBr-KBr;
the proportion of said carbon ranges from 5 percent to 10 percent by weight of V.sub.2 O.sub.5 inclusive;
the proportion of said mixture of alkaline halides ranges from 30 percent to 45 percent by weight of V.sub.2 O.sub.5 inclusive; and
the said heat processing temperature ranges from 410.degree. C. to 550.degree. C., and the duration of the said heat processing ranges from 1/2 hour to 1 hour inclusive.
The resulting compound is then cooled and ground to a particle size of less than 100 microns. It is then pelletized for use as a cathode.
A further object of the invention is a thermal cell comprising the foregoing cathode and, in addition, an anode of lithium or of a lithium alloy selected from the group of compounds consisting of lithium-aluminum, lithium-silicon and lithium-boron, and an electrolyte composed of a binder and of said mixture of alkaline halides.
The binder may be selected from the group consisting of SiO.sub.2, MgO, and Al.sub.2 O.sub.3.
In an unexpected manner, it appears that during the process of fusing the vanadium pentoxide and the mixture of alkaline halides, the gas black exerts a highly specific effect on the constitution of the resulting mixture. This may be due to the fact that the presence of superficial carbon-oxygen bonds formed during the high-temperature heating process subsequently facilitate the transfer of electricity.
During the heat processing period, the mixture of V.sub.2 O.sub.5 and the carbon and the mixture of alkaline halides produces a controlled and reproducible decomposition. It is possible that the carbon also acts as a catalyst during this decomposition process.
The process according to the invention provides for obtaining a complex compound displaying properties which make it well suited for use as the cathode of a thermal cell because:
this complex compound is stable in the operating temperature range applicable to a thermal cell;
when discharging at a constant current, it displays a single discharge stage, which is stable relative to a lithium electrode; this stage being able to be established at +2.5 volts; and
it is well suited for the manufacture of thin pellets.